Donald Trump and Hillary Clinton enter Monday with small, but consistent, leads in polls conducted in the days leading up to the Iowa caucuses. But what if the public polls are wrong?

What if Iowa Republicans choose Ted Cruz or Marco Rubio over Trump on Monday night? What if a surge of Bernie Sanders supporters propels the Vermont senator over Clinton in the Democratic caucuses?

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After a series of high-profile misses in recent elections, America's pollsters outlined the hazards of polling the 2016 Iowa caucuses in interviews with POLITICO over the past week. Here are six things that complicate their efforts and could lead to an official result that doesn't look much like the one the polls appear to predict now.

1. It’s a caucus – not a primary.Participating in a caucus is harder than voting in a primary election. Unlike primaries, caucus voters can only cast their ballots in the evening, at a designated time. And the process is far more time consuming than arriving at the local polling place and pulling the lever. That has implications for turnout.

But what really complicates things for pollsters is the very nature of the caucuses, where voters can and sometime do change their minds during the event. While the process is different for each party, both Democratic and Republican caucuses are communal experiences. Most states don’t allow “electioneering” within polling places, but the caucuses encourage it. Supporters give speeches, making the case for their candidates. In the Democratic caucuses, voters backing candidates who fail to meet a viability threshold in an initial vote – usually 15 percent – are lobbied to pick another candidate in the second round of voting.

Pollsters measure the intentions of voters in the days leading up to the caucuses. Even the entrance polls, which will be cited on all the cable-news networks Monday night, ask caucus-goers their vote preferences when they arrive. But none of that accounts for people who switch to another candidate during the caucus.

J. Ann Selzer, the long-time pollster behind the storied Des Moines Register survey, said last week that lobbying does have an impact.

“You may be someone who is for John Kasich, and you get approached by 17 Jeb Bush people” at the caucus site, she said. “All a poll can do is estimate what people intend to do,” Selzer said. “But it’s a process designed for people to change their minds in the room.”

2. Who’s in the poll?Pollsters call the caucuses low-incidence events. Attendance at the 2012 GOP caucus was only 6 percent of the overall pool of registered voters in the state. Four out of every five registered Republicans didn’t participate four years ago.

But when pollsters call voters about their intentions before an election, those voter lie – or, more charitably, overestimate their own plans to vote. So pollsters are faced with the challenge of weeding out those who won’t actually show up on Caucus Night.

It’s a decision they often make before making a single phone call. Some pollsters choose to dial phone numbers in the state at random, so that every Iowan has an equal chance of being contacted. But other pollsters argue that that sampling frame is too broad. They restrict their surveys to those with a history of past turnout in caucuses, primaries and general elections.

More active voters are more honest about their own chances to vote than those who have a spottier voting record, according to Patrick Murray, the director of the Monmouth University Polling Institute. “People who have a higher proclivity of voting – if they say they are more likely to show up this time, they are more likely to show up this time. If they say they are less likely to show up this time, they are less likely to show up this time,” Murray said. “They are less likely to lie.”

There’s a middle ground, including how Selzer conducts polls for the Register and Bloomberg Politics: Call all registered voters in an effort to exclude non-voters – but not to cut out those who might be more motivated this year than in the past.

3. Trump and Sanders might change voter profile.Getting that right mix of voters is complicated in many ways by Trump and Sanders, who are appealing to non-traditional caucus-goers: Trump has a 16-point lead among respondents to the Des Moines Register/Bloomberg poll who say they are participating in their first Republican caucus, while Sanders leads first-time Democratic caucus-goers by 19 points.

That’s why it’s widely thought that a surge in turnout would boost those two men – and hurt Ted Cruz and Hillary Clinton, respectively. But will those people actually show up on Caucus Night?

Of course, pre-election polls can’t determine that. But some are already arguing that the polls are including too many voters who won’t turn out on Monday, usually using some back-of-the-envelope math.

Jeff Roe, Cruz’s campaign manager, tweeted Saturday night after the release of the Register survey that Selzer’s poll was making assumptions about turnout that were unlikely to be borne out. Roe pointed out that the poll interviewed 3,019 registered voters – 602 of whom were determined to be likely to attend a Republican caucus.

But all registered voters isn’t necessarily equivalent to those registered voters who would participate in a poll. Studies show that voters who are more likely to take the time to answer pollsters’ questions are also more likely to turn out for an election.

For her part, Selzer said her poll isn’t intended to be a turnout estimate. “I don’t think polling is a good tool for measuring turnout,” Selzer said in a separate interview late last year. “Polling is an estimation tool. So if we think there’s going to be a difference of 20,000 voters out of a state of 2.1 million, you’re dealing with such small fractions of percentage points that would make a difference.”

4. Turnout's king but it's hard to predict.While the polls themselves aren’t necessarily indicative of what turnout will be on Monday night, turnout will affect the post-game verdict on whether those polls successfully predicted the outcome in Iowa.

Monmouth University polling data released last week looked at the horse-race at various turnout levels and showed Trump and Sanders both performing best with greater numbers of voters participating. Lower overall turnout, on the other hand, would appear to benefit Cruz and Clinton, respectively.

In the age of big-data campaigning, each candidate’s team knows which turnout levels are helpful or unhelpful for them – and they are acting accordingly, trying to motivate voters more likely to support them on Monday night.

But there are a number of other factors, too. There are the actions of the other candidates, and then there are external dynamics, like a snowstorm expected across much of Iowa on Tuesday that could begin Monday night.

5. Democrats count delegates, not votes.The polls of the Democratic caucuses could be wrong – either Clinton or Sanders could romp to a decisive victory. But that will be harder to evaluate than on the Republican side.

That’s because, while Republicans report the total number of votes tallied by each candidate, Democrats only release the number of delegates won by each candidate at every precinct.

It’s possible to estimate vote totals by multiplying the percentage of delegates won by the candidates by the total number of attendees, but that’s imprecise. And, in many precincts, it’s likely any supporters of former Maryland Gov. Martin O’Malley will be up for grabs, as O’Malley appears unlikely to meet the viability thresholds in the initial balloting.

“We are predicting something,” said Monmouth’s Murray, “where we’ll never know if we’re right or wrong.”

6. Polling is harder now.Recent years haven’t been kind to political pollsters. And continuing technological changes in the ways in which Americans communicate mean 2016 could be even more difficult than other elections in which the polls were wrong.

That means – in addition to trying to thread the needle and accurately predict the volatile caucuses – pollsters have to grapple with all these changes.

Asked by POLITICO last fall if polling this election was more difficult than in the past, Selzer said: “Yes, but I would have said that four, eight, 12 years ago. At every given point it’s more difficult than it was in the good ole’ days of George Gallup.”